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Design >> Analog Design >> about twin-t filter parameter calculation
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Message started by lhlbluesky_lhl on Jan 19^th, 2013, 10:37pm

Title: about twin-t filter parameter calculation
Post by lhlbluesky_lhl on Jan 19^th, 2013, 10:37pm

below is the transfer function of twin-t filter. in actual design, it can also be used for bandpass filter if connected between negative input and output of opamp. now, the twin-t network is a three order system, so, how to get the expression of center frequency f0 and quality factor Q? for two order system, we use 's^2+(2*pi*f0/Q)*s+(2*pi*f0)^2' to calculate f0 and Q, but for the three order system, how to calculate the parameter f0 and Q?
thanks all.

Title: Re: about twin-t filter parameter calculation
Post by raja.cedt on Jan 19^th, 2013, 10:56pm

hello,
this is not a Band-pass Filter(at s=0 and s=inf gain=1) transfer function..

Title: Re: about twin-t filter parameter calculation
Post by buddypoor on Jan 20^th, 2013, 1:56am

raja.cedt wrote on Jan 19^th, 2013, 10:56pm:

hello,
this is not a Band-pass Filter(at s=0 and s=inf gain=1) transfer function..

It is.
Ihlbluesky spoke about a twin-T section in the feedback path of an opamp.

If you choose C3=C1+C2 and at the same time R3=R1||R2 you have a pole-zero pair that cancels each other.
Thus, there is a reduction in order from n=3 to n=2.
Therefore, the 2nd order denominator allows to calculate the Q and wp expressions.

Title: Re: about twin-t filter parameter calculation
Post by lhlbluesky_lhl on Jan 20^th, 2013, 4:02am

but if the relationship C1=C2+C3 and R3=R1||R2 is not satisfied (there is mismatch), such as R1=R2=2*R3 and C2=C3=a*C1(a is not 1/2), so, how to get the expression of f0 and Q? or is there some simple expression here?